Driving circuit for driving a led array

ABSTRACT

A control chip of a driving circuit for driving a LED array shares a ground terminal with the LED array so that, without an additional winding, the driving circuit can provide a supply voltage for the control chip, implement a zero-current switching function, and implement an over-voltage protection function. Since no additional windings are needed, the related costs and the size of the driving circuit are decreased.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/047,154, filed Feb. 18, 2016, which claims priority to Chinese PatentApplication No. 201510124204.0, filed Mar. 20, 2015, and all thebenefits accruing therefrom under 35 U.S.C. §119, the contents of whichin its entirety are herein incorporated by reference.

FIELD OF THE INVENTION

The present invention is related generally to a driving circuit fordriving a light emitting diode (LED) array and, more particularly, to aLED driving circuit that does no need an additional winding.

BACKGROUND OF THE INVENTION

Generally, a driving circuit of a LED array usually utilizes a criticalconduction mode (CRM) or quasi-resonant (QR) buck-boost converter. Suchbuck-boost converter needs an additional winding for providing a currentto a control chip, thereby implementing a zero-current switchingfunction. FIG. 1 shows a conventional driving circuit of the LED array.An alternating current (AC) voltage VACIN is rectified by a rectifier 15so as to generate a direct current (DC) voltage Vin. A control chip 13controls a transistor Q1 to be turned on or turned off so as to generatea stable output voltage Vo or a stable output current Io, so that theLED array 11 can be driven. Referring to FIG. 1, a ground terminal ofthe LED array 11 is different from that of the control chip 13.Accordingly, the control chip 13 is unable to detect the voltage and thecurrent on the LED array 11 directly. Thus, an additional winding N2 isrequired in order to implement the following functions:

1.) providing a current Ivdd to charge a capacitor CVDD so as to providea supply voltage to the control chip 13;

2.) zero-current switching function; and

3.) over-voltage protection function of the output voltage.

FIG. 2 is a waveform diagram of the voltages and the currents in thecircuitry shown in FIG. 1, in which the waveform 17 represents a voltageVds of a drain terminal of the transistor Q1, the waveform 19 representsa voltage VAUX on the winding N2, and the waveform 21 represents acurrent Idout on a diode Dout. The current Ivdd provided by the windingN2 in the circuitry of FIG. 1 will charge the capacitor CVDD, therebymaintaining the supply voltage VDD of the control chip 13. When thetransistor Q1 is turned off, the voltage VAUX on the winding N2 isproportional to the voltage Vds of the drain terminal of the transistorQ1, as shown by waveforms 17 and 19. Simultaneously, a voltage on awinding N1 is almost the same as the output voltage Vo. Thus, thevoltage VAUX is also proportional to the output voltage Vo. ResistorsRzcd1 and Rzcd2 divide the voltage VAUX to generate the voltage Vd to apin ZCD of the control chip 13. The control chip 13 is able to judge thevalue of the output voltage Vo via the voltage Vd, thereby achieving theover-voltage protection of the output voltage Vo. Referring to waveform21 in FIG. 2, when the current Idout on the diode Dout decreases tozero, the voltage VAUX on the winding N2 generates a resonance, whichcan be utilized to implement the zero-current switching of the currentIdout. However, the additional winding N2 will cause a higher cost andincrease the size of the driving circuit board.

Therefore, it is desired a LED driving circuit that needs no additionalwinding.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a driving circuitthat is able to drive a LED array without an additional winding.

According to the present invention, a driving circuit for driving a LEDarray comprises a positive voltage output terminal and a negativevoltage output terminal to be respectively coupled to an anode and acathode of the LED array, a rectifier for rectifying an AC voltage togenerate a DC voltage, a transistor coupled to the rectifier and acircuit ground terminal, a capacitor coupled between the positivevoltage output terminal and the negative voltage output terminal, aninductor coupled to the capacitor via a diode, and a control chip thatincludes a first pin coupled to the transistor, a second pin coupled tothe circuit ground terminal, and a third pin configured to operablydetect a voltage of the inductor. The first pin of the control chipprovides a switching signal for controlling switching of the transistor,so that a stable output voltage or a stable output current can begenerated between the positive voltage output terminal and the negativevoltage output terminal. When the transistor is turned off, a current isprovided by the inductor so as to charge the capacitor. The third pin ofthe control chip directly detects the voltage of the inductor to acquirea discharging time of the inductor and the voltage between the positivevoltage output terminal and the negative voltage output terminal forimplementing a zero-current switching function and an over-voltageprotection function respectively. Additionally, when the transistor isturned off, the LED array can provide a current to charge a power supplycapacitor, thereby providing the supply voltage to the control chip.

The driving circuit of the present invention requires no additionalwinding but still provides the supply voltage, and the zero-currentswitching function and the over-voltage protection function can be alsoimplemented. Therefore, the related costs can be decreased, and the sizeof the driving circuit board is smaller.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objectives, features and advantages of the presentinvention will become apparent to those skilled in the art uponconsideration of the following description of the preferred embodimentsaccording to the present invention taken in conjunction with theaccompanying drawings, in which:

FIG. 1 shows a conventional driving circuit for driving a LED array;

FIG. 2 is a waveform diagram of voltages and currents in the circuitryof FIG. 1;

FIG. 3 shows a circuitry of a first embodiment of a driving circuit fordriving a LED array according to the present invention;

FIG. 4 is a waveform diagram of voltages and currents in the circuitryof FIG. 3; and

FIG. 5 shows a circuitry of a second embodiment of a driving circuit fordriving a LED array according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 shows a first embodiment of a driving circuit 23 according to thepresent invention, which includes a positive voltage output terminal 25and a negative voltage output terminal 27 coupled to an anode 311 and acathode 313 of a LED array 31, respectively. In the driving circuit 23,a rectifier 15 rectifies an AC voltage VACIN for generating a DC voltageVin. A drain of a transistor Q1 is coupled to a capacitor Cin and therectifier 15 for receiving the DC voltage Vin. A current sensingresistor RCS is coupled between a source of the transistor Q1 and acircuit ground terminal 35. An inductor L has a first terminal 37coupled to the anode 311 of the LED array 31, and a second terminal 39coupled to the circuit ground terminal 35. The cathode 313 of the LEDarray 31 is coupled to the circuit ground terminal 35 via a diode Dout.The diode Dout avoids reverse current from the positive voltage outputterminal 25 to the first terminal 37 of the inductor L when the outputvoltage Vo is higher than a voltage VL of the inductor L. A capacitorCout is coupled between the positive voltage output terminal 25 and thenegative voltage output terminal 27. A first pin GD of the control chip33 is coupled to a gate of the transistor Q1 and provides a switchingsignal Vg to control switching of the transistor Q1. Accordingly, astable output voltage Vo or a stable output current Io can be generatedbetween the positive voltage output terminal 25 and the negative voltageoutput terminal 27. Since the brightness of a LED is proportional to thecurrent flowing through the LED, a constant current is generallyutilized to drive a LED. When the transistor Q1 is turned on, a currentIq1 flows through the resistor Rcs from the capacitor Cin to theinductor L to charge the inductor L. When the transistor Q1 is turnedoff, the inductor L provides a current Idout to charge the capacitorCout. A second pin GND of the control chip 33 is coupled to the circuitground terminal 35. A voltage of the second pin GND determines a groundpotential of the control chip 33. Resistors Rzcd1 and Rzcd2 divide thevoltage VL on the inductor L to generate a voltage Vd applied to a thirdpin ZCD of the control chip 33. A fourth pin VDD of the control chip 33is coupled to the power supply capacitor CVDD. The LED array 31 providesa current Ivdd to charge the power supply capacitor CVDD via the diodeDaux. Accordingly, a supply voltage can be provided to the control chip33. Wherein, the diode Daux avoids reverse current from the capacitorCVDD to the LED array 31. In the embodiment shown in FIG. 3, the currentIvdd will be drawn from an anode of one of the LEDs in the LED array 31.Alternatively, the diode Dout in the embodiment shown in FIG. 3 can becoupled between the first terminal 37 of the inductor L and the positivevoltage output terminal 25.

FIG. 4 is a waveform diagram of the voltages and currents in thecircuitry shown in FIG. 3, in which the waveform 41 represents thevoltage VL and the waveform 43 represents the current Idout. When thetransistor Q1 becomes off from the on state as shown at time t1, theinductor L starts discharging to provide the current Idout for chargingthe capacitor Cout. In the same time, the LED array 31 also provides thecurrent Ivdd to charge the power supply capacitor CVDD. When theinductor provides the current Idout as shown from time t1 to time t2,the voltage VL on the inductor L equals the output voltage Vo, and thevoltage VL is almost maintained at a fixed value. The resistors Rzcd1and Rzcd2 divide the voltage VL to generate the voltage Vd, which isrelated to the output voltage Vo, to apply to the third pin ZCD of thecontrol chip 33. Herein, the diode Dout is on, so the second pin GND ofthe control chip 33 and the negative voltage output terminal 27 (or anegative terminal of the capacitor Cout) are coupled to the same circuitground terminal 35. Thereby, the control chip 33 is able to judge thevalue of the output voltage Vo according to the voltage Vd directly, soas to implement the over-voltage protection when the output voltage Vois too high. When the current Idout decreases to zero as shown at timet2, the voltage VL of the inductor L generates a resonance and startsdecreasing. As a result, the voltage Vd also decreases. When the voltageVd becomes lower than a preset value, the current Idout is regarded asdecreasing to zero by the control chip 33. That is to say, the controlchip 33 can directly detect the discharging time of the inductor L fromthe voltage VL of the inductor L, thereby implementing the zero-currentswitching function. Generally speaking, the zero-current switching willturn on the transistor Q1 immediately when the current Idout decreasesto zero. In some applications, the transistor will not be turned onuntil the current Idout has decreased to zero for a while. For example,as shown by the waveform 41 in FIG. 4, the transistor Q1 will not beturned on until the voltage VL decrease to a valley value as shown attime t3, thereby achieving better performance.

FIG. 5 shows a second embodiment of the driving circuit 23 according tothe present invention. The circuitry shown in FIG. 5 is almost the sameas that in FIG. 3, while the diode Dout in this embodiment is coupledbetween the first terminal 37 of the inductor L and the positive voltageoutput terminal. The locations of the diodes Dout in the embodiments ofFIG. 5 and FIG. 3 are different, but both diodes Dout can preventreverse current. Another difference between the circuitries in theembodiments of FIG. 5 and FIG. 3 is that the current Ivdd in FIG. 5 isdrawn from the anode 311 of the LED array 31. The current Ivdd in FIG. 5also charges the capacitor CVDD to generate the supply voltage to thecontrol chip 33. In order to prevent the supply voltage of the controlchip 33 over a permissible range due to the output voltage Vo, a Zenerdiode DZ can be coupled between the anode 311 of the LED array 31 andthe capacitor CVDD so as to clamp the supply voltage of the control chip33. Wherein, the diode Dout in FIG. 5 can be also coupled between thesecond terminal 39 of the inductor L and the negative voltage outputterminal 27.

The driving circuit 23 of the present invention doesn't need anadditional winding and can implement the over-voltage protectionfunction of the output voltage, implement the zero-current switchingfunction, and provide the current Ivdd to the power supply capacitorCVDD. Accordingly, the related costs and the size of the driving circuitboard can be decreased.

While the present invention has been described in conjunction withpreferred embodiments thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and scopethereof as set forth in the appended claims.

What is claimed is:
 1. A driving circuit for driving a light emittingdiode (LED) array, comprising: a positive voltage output terminal to becoupled to an anode of the LED array; a negative voltage output terminalto be coupled to a cathode of the LED array; a rectifier configured tooperably rectify an alternating current (AC) voltage to generate adirect current (DC) voltage; a transistor coupled to the rectifier; acircuit ground terminal; a capacitor coupled between the positivevoltage output terminal and the negative voltage output terminal; adiode; an inductor having a first terminal and a second terminal,coupled to the capacitor via the diode, and configured to operablyprovide a current to charge the capacitor when the transistor is turnedoff; and a control chip having a first pin coupled to the transistor andconfigured to operably provide a switching signal for controllingswitching of the transistor so as to generate a stable output voltagebetween the positive voltage output terminal and the negative voltageoutput terminal, a second pin coupled to the circuit ground terminal,and a third pin configured to operably detect a voltage of the inductor;wherein, when the transistor is turned off, the control chip willdirectly detect the voltage of the inductor to acquire a dischargingtime of the inductor for implementing a zero-current switching function,or to acquire the output voltage for implementing an over-voltageprotection function.
 2. The driving circuit of claim 1, wherein a drainof the transistor receives the DC voltage and a source of the transistoris coupled to the inductor via a current sensing resistor.
 3. Thedriving circuit of claim 1, wherein a supply voltage of the control chipis provided by the LED array.
 4. The driving circuit of claim 1, whereinthe second pin of the control chip is coupled to the negative voltageoutput terminal.
 5. The driving circuit of claim 1, further comprising apower supply capacitor coupled to the control chip and to be charged bya current from the LED array when the transistor is turned off.
 6. Thedriving circuit of claim 1, wherein the first terminal of the inductoris coupled to the positive voltage output terminal and the secondterminal of the inductor is coupled to the circuit ground terminal. 7.The driving circuit of claim 1, wherein, when the transistor is turnedoff, a negative terminal of the capacitor is coupled to the circuitground terminal.
 8. The driving circuit of claim 1, wherein, when thetransistor is turned off, the negative voltage output terminal iscoupled to the circuit ground terminal.
 9. A driving circuit for adriving a light emitting diode (LED) array, comprising: a positivevoltage output terminal to be coupled to an anode of the LED array; anegative voltage output terminal to be coupled to a cathode of the LEDarray; a rectifier configured to operably rectify an alternating current(AC) voltage to generate a direct current (DC) voltage; a transistorcoupled to the rectifier; a circuit ground terminal; a capacitor coupledbetween the positive voltage output terminal and the negative voltageoutput terminal; a diode; an inductor having a first terminal and asecond terminal, coupled to the capacitor via the diode, and configuredto operably provide a current to charge the capacitor when thetransistor is turned off; and a control chip having a first pin coupledto the transistor and configured to operably provide a switching signalfor controlling switching of the transistor so as to generate a stableoutput current between the positive voltage output terminal and thenegative voltage output terminal, a second pin coupled to the circuitground terminal, and a third pin configured to operably detect a voltageof the inductor; wherein, when the transistor is turned off, the controlchip will directly detect the voltage of the inductor to acquire adischarging time of the inductor for implementing a zero-currentswitching function, or to acquire a voltage between the positive voltageoutput terminal and the negative voltage output terminal forimplementing an over-voltage protection function.
 10. The drivingcircuit of claim 9, wherein a drain of the transistor receives the DCvoltage and a source of the transistor is coupled to the inductor via acurrent sensing resistor.
 11. The driving circuit of claim 9, wherein asupply voltage of the control chip is provided by the LED array.
 12. Thedriving circuit of claim 9, wherein the second pin of the control chipis coupled to the negative voltage output terminal.
 13. The drivingcircuit of claim 9, further comprising a power supply capacitor coupledto the control chip and to be charged by a current from the LED arraywhen the transistor is turned off.
 14. The driving circuit of claim 9,wherein the first terminal of the inductor is coupled to the positivevoltage output terminal and the second terminal of the inductor iscoupled to the circuit ground terminal.
 15. The driving circuit of claim9, wherein, when the transistor is turned off, a negative terminal ofthe capacitor is coupled to the circuit ground terminal.
 16. The drivingcircuit of claim 9, wherein, when the transistor is turned off, thenegative voltage output terminal is coupled to the circuit groundterminal.